The monaural auditorY evoked response is used as a measure of acute and chronic anticonvulsant effect on central nervous sYstem function. The origin and magnitude of the ipsilateral-contralateral peak latencY difference were defined. Patients before and after initiation of anticonvulsant therapy are being evaluated. Results from a small number of patients allow no definite conclusions, but it appears that phenobarbital gives abnormally high ipsilateral-contralateral peak latency differences.